\$\begingroup\$
You can find more information on our web, so please take a look.
The basic difference is that a pentaprism is a solid block of glass. This means it's fairly heavy. Light enters one side, gets reflected around inside, and then comes out one other side. This means there are only 2 air/glass interfaces involved: one where light enters the prism, and one where it leaves the prism. This keeps light loss to a minimum.
A pentamirror does pretty much the same thing, but it's composed of a number of separate mirrors, so there's air in the middle instead of a block of solid glass. This makes it considerably lighter and less expensive. It also means there's an air/glass interface for each individual mirror, so there's quite a bit more light loss.
As a result, a pentamirror is used almost exclusively in relatively low-end cameras where reduced expense and weight matter more than a relatively dim view through the viewfinder.
Link to optec
what difference does it make in a k-x?
Well, it's not like you get a choice between the two in a K-x - all K-x's have the pentamirror. Compared to a *different* Pentax camera with a pentaprism, the viewfinder is *slightly* smaller and *slightly* dimmer. A $40 O-ME53 magnifier solves the former problem very nicely. Like Dave, I can see the difference in size & brightness if I really look for it, and in theory it might make manual focus slightly easier with a pentaprism camera. In practice, as someone who has owned both, I'd say I don't enough of a difference to really worry. If you plan on mostly using manual lenses, and you have enough experience to know that the small difference would really matter to you, then take that into consideration. Otherwise, if you're just asking because you saw the words in a review and have no idea how relevant it is to you, I'd say it's 99% irrelevant.
If you are looking for more details, kindly visit Penta Prisms.